The present invention relates generally to a write once optical recording medium, and particularly to a write once optical recording medium including an inorganic recording film.
The mainstream of write once optical recording mediums is a write once optical recording medium including a recording film on which organic dye is spin-coated, such as a write once CD (Compact Disc) and a write once DVD (Digital Versatile Disc) as is known as so-called CD-R (Compact Disc-Recordable) and DVD-R (Digital Versatile Disc-Recordable).
Also, there is proposed a storage member including an inorganic recording medium which may meet with the requirements of the CD-R, for example (see Japanese Published Patent Application No. 11-144316, for example).
On the other hand, a blue-ray disc (Blu-ray Disc, BD) which receives a remarkable attention as a next-time optical recording medium is able to realize higher-density information recording and high-vision video recording (high-definition video recording).
The first BD recording apparatus has already been commercially available on the market and this BD recording apparatus may meet with the requirements of the rewritable media using a phase-change material as the recording layer.
It is, however, desired that BD write once media (BD-R) should be requested as the needs of the market and that such BD write once media should become inexpensive as compared with the rewritable media.
According to the standards of the first BD, since the first BD uses the phase-change material as the base material thereof, if the BD-R is designed using inorganic material from the viewpoint of media characteristics such as a push-pull signal recording sensitivity, then it is easy to match the characteristics of the BD-R.
Alternatively, the BD-R may use the phase-change material as the base material thereof and this BD-R is designed so that it is made unable to be re-recorded, whereafter it may meet with the standards of the BD-R. However, according to this arrangement, the BD-R should be initialized. Further, since the film arrangement, the number of films and the materials of this BD-R become equivalent to those of the rewritable media, it is difficult to decrease the cost and it is also difficult to realize inexpensive media.
Above all, the element that considerably affects the cost of the BD-R is the number of films. Therefore, if the number of the films is decreased, then equipment investment of a vacuum evaporation system to deposit films by a sputtering method can be decreased considerably, which may reflect decrease of media cost.
Accordingly, the number of films should preferably be decreased as much as possible and it is desirable that an optical recording medium including three or four layers of which number of lesser than that of the phase-change media should be realized.
In the BD media, the surface from which information may be read out is the opposite side of a substrate, for example, the side of a thin light transmission layer having a thickness of 100 μm which serves as a film covering a recording film. A thickness of this light transmission layer is approximately 70 μm when the BD is a double-layer media. Further, if the standards of multi-layer media are completed, then the above thickness of the light transmission layer may be varied in accordance with the above standards. A range in which the above thickness of the light transmission layer is selected to be 10 μm to 177 μm may be considered as a satisfactory range from the standpoints of skew characteristics and recording and reproducing characteristics.
As methods for forming this light transmission layer, there have been proposed various methods such as a method of bonding a PC (polycarbonate) resin sheet by using a PSA (Pressure Sensitive Adhesive), a method of bonding the PC resin sheet by using a ultraviolet-curing resin, that is, so-called UV resin or a method of forming all of light transmission layers having a thickness of about 100 μm by the UV resin.
Of these methods, the PSA has small hardness as compared with that of the UV resin and hence it may exert large influence on skew characteristics and recording and reproducing characteristics depending on recording materials. Accordingly, stable recording materials and arrangements by which satisfactory recording may be made by using even the PSA are requested.